A fuel cell is a device for converting chemical energy into electricity. An alkaline fuel cell comprises an anode, a cathode and an alkaline electrolyte held within a porous non-conducting matrix between the anode and the cathode. Potassium hydroxide is typically used as the alkaline electrolyte in an alkaline fuel cell. The anode and cathode each comprise a porous substrate and a porous catalyst layer supported on the substrate.
Conventional alkaline fuel cells are operated at temperatures in the range of 65.degree. C. to 200.degree. C. at pressures between 1 and 14 atmospheres. A hydrogen containing gas is fed to the anode and an oxygen containing gas is fed to the cathode. The reactant gases diffuse through the electrodes to react with the electrolyte in the presence of the catalysts to produce water, heat and electricity. At the anode the hydrogen is electrochemically oxidized and gives up electrons according to the reaction: EQU H.sub.2 +2OH.sup.-.fwdarw. 2H.sub.2 O+2e.sup.-.
The electrons so generated are conducted from the anode through an external circuit to the cathode. At the cathode electrons are electrochemically combined with the oxidant according to the reaction: EQU 1/2O.sub.2 O+H.sub.2 O+2e.sup.- 2OH.sup.-.
A flow of ions through the electrolyte completes the electrical circuit.
In a conventional gas diffusion electrode, the catalyst layer comprises a polymeric hydrophobic phase and a wettable catalyst phase. Particles of catalyst form a network of electrolyte- filled channels through the catalyst layer. The above described electrochemical reactions occur at the surfaces of the catalyst particles. The porous hydrophobic phase binds the electrode together and provides a network of channels through which reactant gases gain access to the catalytic surfaces and through which gaseous reaction products escape from the catalyst layer.
The performance of a conventional alkaline fuel cell may be compromised by phenomena termed "flooding" and "pumping". The term "flooding" refers to penetration of liquid into hydrophobic regions of the catalyst layer which should contain only gas. The misplaced liquid hinders and may totally obstruct the supply of reactant gas to local regions of the catalyst. As a result, there is an increase in electrode polarization as non-flooded regions of the electrode are forced to carry more current the flooding phenomenon is most prevalent at the water-producing electrode, i.e. the anode of an alkaline electrolyte fuel cell.
The term "pumping" refers to bulk movement of electrolyte from one side of a fuel cell to the other due to the electromigration of nonelectroactive ions. For example, in an alkaline fuel cell having a KOH electrolyte nonelectroactive potassium ions migrate toward the cathode. During operation at high current densities, pressure may be built up that is sufficient to force electrolyte out of the gas side of the cathode. Accumulation of electrolyte on the gas side of the electrode restricts the supply of reactant gas to the catalyst layer of the electrode and may lead to severe concentration polarization of the electrode.
The conventional approach to the problem of flooding has been to make the catalyst layer of the electrode more hydrophobic. The catalyst layer can be made more hydrophobic by increasing the amount of hydrophobic binder in the layer or by increasing the temperature or process time during sintering of the electrode. The pumping phenomenon has been addressed by applying a thin layer of a porous hydrophobic polymer to the gas side of the electrode. These strategies have achieved only limited success.
What is needed in the art is a way to provide electrodes that are resistant to flooding and pumping and to thereby provide an alkaline fuel cell that may be operated continuously at high current density.